1. Field of the Invention
The subject invention relates to resonant mode power supplies for, e.g., television receivers, and more particularly, to the start up of such resonant mode power supplies.
2. Description of the Related Art
Television receivers include power supplies for supplying power to the various circuit components. Generally, these power supplies are in the form of switched-mode power supplies in which a d.c. voltage is applied to one end of a primary winding of a flyback transformer. The other end of the primary winding is connected to ground through a switching element which is controlled to switch on and off at a determined frequency. The switched power to the transformer is then transferred to the secondary winding for providing the output power.
It has been found that resonant mode power supplies have higher efficiencies and are especially suited for power levels higher than 150W and for power supplies that need to be miniaturized, e.g., power supplies for battery charging. In a resonant mode power supply, an oscillating circuit, which is typically a series oscillating circuit, is supplied through a first switching element with a direct voltage by which the oscillating circuit is excited. After one-half of an oscillating period, the first switching element is opened, and the oscillating circuit is connected to ground through a second switching element, and as a result, the capacitor of the oscillating circuit discharges, and the oscillation is thus continued. An oscillation can accordingly be excited by alternatively opening and closing the switching elements. The output power delivered by a resonant mode power supply is inversely proportional to the oscillation frequency, i.e., the lower the frequency, the higher the output power.
It is customary to configure a resonant mode power supply such that starts up with a soft-start sequence. This is accomplished by arranging the resonant mode power supply, on start-up, to perform a frequency sweep from its maximum value to its minimum value. By sweeping the frequency from the maximum to minimum value, the current delivered to the converter is swept from its minimum value to the maximum peak value. This is done to reduce stress on the system components.
Traditionally, the frequency sweep is performed in a linear fashion using a ramp function. The resonant mode power supply includes a feedback circuit, which generally consists of an opto-controller driven by a capacitor, for controlling the output voltage. As such, this feedback circuit has a slow response time. This results in a large overshoot of the output voltage on start-up before the feedback circuit is able to regulate the output voltage to the desired regulation level. FIG. 1 shows this start-up overshoot behavior. In resonant circuits, due to the larger gain of the circuit close to its operating point (see FIG. 2), this overshoot can be even higher than in a flyback-based power supply.